Talks resume this week in Vienna over Iran's nuclear program. Western powers want to prevent Iran from making nuclear weapons. Iran wants relief from economic sanctions. Well, today, we look at one of the issues: the construction of Iran's heavy-water reactor near the city of Arak. Critics doubt Iran's claims that the reactor is just for medical research, not weapons.

Here's NPR's Peter Kenyon.

PETER KENYON, BYLINE: The Arak reactor is largely separate from the main dispute between Iran and the West over enriching uranium because this type of reactor uses natural uranium, no enrichment needed. The problem from a proliferation standpoint is that this reactor also produces plutonium, which can be used to fuel nuclear weapons.

In fact, former International Atomic Energy Agency director Bob Kelley says decades ago this type of 40 megawatt heavy-water reactor was a popular choice among states with secret nuclear weapons programs.

BOB KELLEY: So, we look at 40 megawatt heavy water reactors - they were used in Israel, India, Pakistan - to produce weapons. And so, when you see one of these, naturally, all the alarm bells go off.

KENYON: Iran says there's no need for alarm bells because this reactor will be used to produce radioisotopes for medical research. But analyst Bob Einhorn, at the Brookings Institution, says this kind of reactor isn't well-suited for that purpose. And that raises suspicions among nuclear non-proliferation experts.

BOB EINHORN: It's not a very good producer of medical isotopes. They would be much better off to convert this to a so-called light-water reactor. It would do the job better. And it would create much less concern because it wouldn't be as efficient a plutonium producer.

KENYON: But as with many issues in these talks, technical fixes sometimes create new problems. In this case, if the West presses Iran to convert Arak into a light-water reactor, it will be helping Iran make the case for keeping its own enrichment program, which Israel and others oppose.

On the other hand, even without converting Arak into a light-water reactor, there are still ways to ensure that no weapons-grade plutonium is produced there. For one thing, Tehran says it won't build a reprocessing plant at Arak, which is necessary to extract the plutonium.

But even if the West isn't willing to take Tehran's word for that, analyst Bob Kelley says regular IAEA inspections would be able to determine whether or not the reactor is capable of producing weapons-grade fuel.

KELLEY: If the fuel stays in the reactor for more than about six or eight weeks, it gets cooked to a point where it's no longer useful for weapons - it spoils it. And so, if the reactor is operated on very short cycles, IAEA will tell you: Yeah, they're making weapons-grade plutonium. But if they refuel the reactor, let's say once a year, the material is no good for weapons and the IAEA's presence will verify that.

KENYON: But a number of officials question the very need for such a reactor in a modern, peaceful nuclear program. And critics of the Islamic republic ask why Tehran insists on finishing it if it doesn't want the plutonium it could produce. Kelley says there could be a number of answers to that question, including the old standby, money.

KELLEY: If I am the Revolutionary Guard of Iran and I plan to build a $500 million plant and everybody I know is going to get a kickback and some of the profit from building this plant, I'm going to finish it. And so that's probably what's happened and now there's just a certain stubbornness and pride in saying we need them.

KENYON: Even so, this seemingly isolated issue has already complicated these nuclear talks and could do so again.